Conventionally, an air-blowing device is known to have an axial fan supplying air to a radiator and a shroud that supports the axial fan and forms an air passage extending from the radiator to the axial fan.
Generally, in this kind of air-blowing device, the shroud has a rectangular shape in a plane view to fit the radiator having a rectangular shape. Accordingly, a length (that will be referred to as a length of an air-introduction part) between an outer end portion of the axial fan in a radial direction and a periphery of the shroud becomes partially short or partially long. In a part where the length of the air-introduction part is long, an attack angle decreases since an air volume flowing into the axial fan increases. On the other hand, in a part where the length of the air-introduction part is short, the attack angle increases since the air volume flowing into the axial fan decreases.
Therefore, a negative pressure caused on a surface of a blade of the axial fan varies between in the part where the length of the air-introduction part is long and in the part where the length of the air-introduction part is short, and changes alternately. This variation of the pressure waves air and transmits as a sound wave. According to the above-described shroud having a rectangular shape in a plane view, an order sound-component that is synchronized with a rotation of the axial direction increases, and a noise may increase, since the sound wave has a periodicity.
On the other hand, in an air-blowing device described in Patent Literature 1, by providing an opening in an air passage of a shroud on an upstream side of an axial fan, an unbalance of pressure is canceled and a first-order and second-order rotational noises are reduced. Furthermore, in the air-blowing device, by providing a current plate that protrudes inward from an inner surface of the shroud located on an upstream side of the axial fan, a second-order and fourth-order rotational noises are also reduced.